Depression isn’t a yetzer hara. It is a serious medical, psychological, and emotional condition, and like other mental illnesses, there is a very strong spiritual dimension, as well.
R’ Twerski is a medical doctor, and for him to not recommend psychotropic drugs would constitute malpractice on his part, when individual patients require such medication.
If anything, there may be a strong yetzer hara for individuals not to seek out the proper medical assistance. The yetzer hara convinces us there’s nothing seriously wrong, when in fact, we do have serious problems that require professional attention and treatment.